Presentation

30 year-old female presented to the emergency department following inversion injury to left ankle. There is focal tenderness and swelling over lateral malleolus. Patient reports history of recurrent left ankle sprain and instability.

Patient Data

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Frontal

Anteriorposterior ankle radiograph demonstrates a subtle lucent region in the lateral facet of talus (red arrow). An unequivocal wedge radiodensity is present in the adjacent soft tissue (blue arrow). There is soft tissue swelling adjacent to the lateral malleolus. Patient was referred for CT study for further characterization.

There is subchondral sclerosis of the medial facet of the talus and the corresponding medial malleolus. A well corticated triangular bone fragment is present beneath the medial malleolus. The finding is in keeping with previous impaction injury between the medial malleolus and medial facet of talus. The likely mechanism is an inversion and adduction injury. The corticated triangular bone fragment is likely to represent an old fracture. Less likely differential diagnosis would include an accessory ossicle (os subtibiale) which usually has a rounded / ovoid appearance.

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Coronal bone window

Coronal CT images demonstrate bone avulsion from the lateral facet of the talus.

Axial CT images show a irregular bone fragment in the anteroinferior aspect of the lateral malleolus. There is also elevation of the periosteum of the head of fibula. The location and imaging findings is suggestive of anterior talofibular ligament injury with an avulsed bone fragment from talus.

Case Discussion

The prevalence of bone avulsion from anterior talofibular ligament (ATFL) injury may be higher than previously thought because the avulsed bone fragment may only be faintly visible or not visible at all on ankle radiographs. Operative findings of acute sprained ankles found that 12% of ATFL injury had avulsed bone fragment 1. Haraguchi et al in an anatomical study suggest two new radiographic projection for evaluating suspected avulsion fractures of the lateral malleolus 2. The management of subtle avulsion fracture from ATFL injury is unlikely to be different to an isolated ATFL ligamentous injury.